The overall objective of this study is to perform a mechanistic analysis of cardiovascular regulation during orthostasis. The goal is to achieve an indepth understanding of how individuals become predisposed to orthostatic hypotension after simulated microgravity, i.e. bedrest. The study will also evaluate changes in central hemodynamics and cardiac mechanics, as well as, peripheral regulation of blood flow and vascular resistance in normal human subjects before and after short and long term bedrest.